In particular the present invention relates to the oxygen linear sensor test arrangement known as the "UEGO" sensor (universal exhaust gas oxygen sensor), to which the following makes explicit reference though without loss of its general nature.
The present invention is of advantage in the automotive field, where it is known to use a UEGO sensor, located along the exhaust manifold of an internal combustion engine, to obtain information concerning the composition of the exhaust gases.
The UEGO sensor has two electrolytic cells responding to oxygen ions, and a diffusion chamber placed between the actual cells and able to receive part of the combustion gases at the outlet from the engine. In addition, the UEGO sensor provides for the use of a test arrangement which is connected to the sensor itself by means of a connector, and is able to control electrically the electrolytic cells to produce a test action on the sensor itself. In particular such control means is designed to provide an oxygen ion draining mechanism from the diffusion chamber to atmosphere and vice versa, in order to obtain within the actual chamber an exhaust gas composition equal to that which would obtain in the event of the air/petrol ratio of the mixture fed to the engine becoming stoichiometric, i.e. equal to 14.57.
The intensity of the test action, that is to say the intensity of the pumping current required to maintain the stoichiometric level within the diffusion chamber, is the information on the basis whereof the test arrangement generates the Vout output signal representative of the composition of the exhaust gases proceeding from the engine. The Vout signal is proportional to the quantity of oxygen present within the exhaust gas and consequently, is indicative of the air stroke/petrol ratio of the mixture fed to the engine.
Presently the test arrangements, before installation on the vehicle, need to be connected to a compensation resistance to compensate for eventual spreads (variation) of the pumping current.
The compensation resistance, the nominal value whereof is shown by the sensor manufacturer on completion of production, is inserted between two connections of the said connector, and cooperating with the test arrangement, actively intervenes in the generation of the Vout signal. Unfortunately the compensation resistance, being located within the engine space, is subject to heavy thermal stresses during operation of the engine.
In consequence, on variation of the engine temperature it occurs that the compensation resistance assumes differing values in relation to the nominal value which would ensure a correct compensation. This means that the variation in pumping currents are not adequately compensated and that the control arrangement provides a Vout output signal which is not effectively indicative of the composition of the exhaust gases.